The present invention relates to a ferroelectric film and a semiconductor device including the ferroelectric film.
Ferroelectric films have been widely used in a variety of semiconductor devices, such as a ferroelectric capacitor having first and second electrodes formed on respective faces of a ferroelectric film, an MFS transistor having an MFS (Metal/Ferroelectric/Semiconductor) multilayer structure, an MFIS transistor having an MFIS (Metal/Ferroelectric/Insulator/Semiconductor) multilayer structure and an MFMIS transistor having an MFMIS (Metal/Ferroelectric/Metal/Insulator/Semiconductor) multilayer structure.
Known ferroelectric materials used for forming a ferroelectric film are a PZT (PbZrxTi1xe2x88x92xO3)-based ferroelectric material and a Bi-based ferroelectric material such as SBT (SrBi2Ta2O9) or BTO (Bi4Ti3O12), and a BTO-based ferroelectric material is regarded as a promising material because it can exhibit a good ferroelectric characteristic.
In particular, as disclosed in, for example, Japanese Laid-Open Patent Publication No. 2000-260960, a ferroelectric material obtained by substituting a nonvolatile element A for part of Bi of Bi4Ti3O12 and represented by a general formula, Bi4xe2x88x92xAxTi3O12 (wherein A is selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; and 0 less than x less than 2.8) has been recently proposed.
Japanese Laid-Open Patent Publication No. 2000-260960 describes that a ferroelectric film can be obtained by sintering a film made from the ferroelectric material represented by the general formula, Bi4xe2x88x92xAxTi3O12 in an oxygen atmosphere for approximately 1 hour at a temperature of approximately 700xc2x0 C.
The present inventors have found, however, that when a film made from the ferroelectric material represented by the general formula, Bi4xe2x88x92xAxTi3O12 is sintered in an oxygen atmosphere at approximately 700xc2x0 C., the resultant ferroelectric film has a problem in its reliability although its initial characteristic is good. Specifically, the present inventors have found that when a semiconductor device including the ferroelectric film represented by the general formula, Bi4xe2x88x92xAxTi3O12 is operated for approximately 10 years or is made to repeatedly perform a rewrite operation at a temperature of 85xc2x0 C. or more, the ferroelectric film cannot keep its polarization characteristic.
Furthermore, in order to realize a high degree of integration of a semiconductor device including a ferroelectric film, both compactness and a low-voltage operating characteristic are required. A ferroelectric film made from the ferroelectric material represented by the general formula, Bi4xe2x88x92xAxTi3O12 has, however, a problem in the low-voltage operating characteristic. Therefore, a semiconductor device including a ferroelectric film made from the ferroelectric material represented by the general formula, Bi4xe2x88x92xAxTi3O12 is disadvantageous in operating at a low voltage and realizing a high degree of integration.
In consideration of the aforementioned conventional disadvantages, an object of the invention is improving the reliability and the low-voltage operating characteristic of a ferroelectric film made from a ferroelectric material represented by a general formula, Bi4xe2x88x92xAxTi3O12.
The first finding based on which the present invention was devised is as follows:
The present inventors have examined the reason why a ferroelectric film represented by the general formula, Bi4xe2x88x92xAxTi3O12 is degraded in its polarization characteristic when a semiconductor device including the ferroelectric film is operated for approximately 10 years or is made to repeatedly perform a rewrite operation at a temperature of 85xc2x0 C. or more. As a result, it has been found that the polarization characteristic is degraded because Bi escapes from the crystal structure with the elapse of time.
The second finding is as follows:
The present inventors have examined the reason why a ferroelectric film represented by the general formula, Bi4xe2x88x92xAxTi3O12 is difficult to operate at a low voltage. As a result, it has been found that a ferroelectric film can be operated at a low voltage by reducing an interatomic distance between Ti and O in the crystal structure. Specifically, when the interatomic distance between Ti and O is large, large energy, that is, a high voltage, is necessary for moving O to cause polarization, but when the interatomic distance between Ti and O is small, energy, namely, a voltage, necessary for moving O to cause polarization is small, and therefore, the ferroelectric film can be thus operated at a low voltage.
The first aspect of the invention is on the basis of the first finding, and Bi is excessively included in a ferroelectric material represented by the general formula, Bi4xe2x88x92xAxTi3O12.
Specifically, the first ferroelectric film of the invention is made from a ferroelectric material represented by a general formula (1), Bi4xe2x88x92x+yAxTi3O12, wherein A is an element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and V; and 0xe2x89xa6xxe2x89xa62 and 0 less than yxe2x89xa6(4xe2x88x92x)xc3x970.1.
In the first ferroelectric film of the invention, since Bi is included by an excessive amount corresponding to a composition ratio y (y greater than 0) of the general formula (1), even though Bi escapes from the crystal structure with the elapse of time, the excessive Bi enters a portion from which Bi has escaped, and therefore, the characteristic of the ferroelectric film is never degraded. Also, the Bi excessive content y satisfies a relationship of yxe2x89xa6(4xe2x88x92x)xc3x970.1, a leakage current flowing due to deposition of bismuth on the ferroelectric film can be prevented. Accordingly, the reliability of the ferroelectric film can be improved.
Preferably, in the first ferroelectric film of the invention, (4xe2x88x92x)xc3x970.02xe2x89xa6yxe2x89xa6(4xe2x88x92x)xc3x970.06 in the general formula (1).
Thus, the initial polarization value of the ferroelectric film can be 20 xcexcC/cm2 or more, and hence, the reliability of the ferroelectric film can be improved.
Preferably, in the first ferroelectric film of the invention, A is La and 0.65xe2x89xa6xxe2x89xa60.85 in the general formula (1).
Thus, the initial polarization value of the ferroelectric film can be 20 xcexcC/cm2 or more, and hence, the reliability of the ferroelectric film can be improved.
The second aspect of the invention is on the basis of the second finding. Specifically, the second ferroelectric film of the invention is made from a ferroelectric material represented by a general formula (2), (Bi4xe2x88x92x+yAxTi3O12)z+(DBi2E2O9)1xe2x88x92z, wherein A is an element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and V; D is an element selected from the group consisting of Sr, Ba, Ca, Bi, Cd, Pd and La; E is an element selected from the group consisting of Ti, Ta, Hf, W, Nb, Zr and Cr; and 0xe2x89xa6xxe2x89xa62, 0 less than yxe2x89xa6(4xe2x88x92x)xc3x970.1 and 0.5 less than z less than 1.
In the second ferroelectric film of the invention, since Bi is included by an excessive amount corresponding to a composition ratio y (y greater than 0) of the general formula (2), even though Bi escapes from the crystal structure with the elapse of time, the excessive Bi enters a portion from which Bi has escaped, and therefore, the characteristic of the ferroelectric film is never degraded. Also, the Bi excessive content y satisfies a relationship of yxe2x89xa6(4xe2x88x92x)xc3x970.1, a leakage current flowing due to deposition of bismuth on the ferroelectric film can be prevented. Accordingly, the reliability of the ferroelectric film can be improved.
Also, since the ferroelectric material is represented by the general formula (2), (Bi4xe2x88x92x+yAxTi3O12)z+(DBi2E2O9)1xe2x88x92z, the ferroelectric material has a superlattice structure in which (DBi2E2O9) operable at a lower voltage than (Bi4xe2x88x92xAxTi3O12) is substituted for part of (Bi4xe2x88x92xAxTi3O12). Therefore, a Ti atom included in (Bi4xe2x88x92xAxTi3O12) is substituted with an element E of the general formula having a smaller interatomic distance to an O atom than the Ti atom, so that a low-voltage operation of the ferroelectric film can be realized.
The third ferroelectric film of this invention is made from a ferroelectric material represented by a general formula (3), Bi4xe2x88x92x+yxe2x88x92rAxDrTi3xe2x88x92sEsO12, wherein A is an element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and V; D is an element selected from the group consisting of Sr, Ba, Ca, Cd, Pb and La; E is an element selected from the group consisting of Ta, Hf, W, Nb, Zr and Cr; and 0xe2x89xa6xxe2x89xa62, 0 less than yxe2x89xa6(4xe2x88x92x)xc3x970.1, 0 less than x+rxe2x89xa62, 0xe2x89xa6rxe2x89xa60.5 and 0xe2x89xa6sxe2x89xa60.5.
In the third ferroelectric film of the invention, since Bi is included by an excessive amount corresponding to a composition ratio y (y greater than 0) of the general formula (3), even though Bi escapes from the crystal structure with the elapse of time, the excessive Bi enters a portion from which Bi has escaped, and therefore, the characteristic of the ferroelectric film is never degraded. Also, the Bi excessive content y satisfies a relationship of yxe2x89xa6(4xe2x88x92x)xc3x970.1, a leakage current flowing due to deposition of bismuth on the ferroelectric film can be prevented. Accordingly, the reliability of the ferroelectric film can be improved.
Also, since the ferroelectric material is represented by the general formula (3), Bi4xe2x88x92x+yxe2x88x92rAxDrTi3xe2x88x92sEsO12, a Ti atom included in Bi4xe2x88x92x+yAxTi3O12 is substituted with an element E of the general formula (3), namely, an element having a smaller interatomic distance to an O atom than the Ti atom, so that the low-voltage operation of the ferroelectric film can be realized.
The first semiconductor device of this invention includes a ferroelectric film; and an electrode provided in contact with at least one face of the ferroelectric film, and the ferroelectric film is made from a ferroelectric material represented by a general formula (1), Bi4xe2x88x92x+yAxTi3O12, wherein A is an element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and V; and 0xe2x89xa6xxe2x89xa62 and 0 less than yxe2x89xa6(4xe2x88x92x)xc3x970.1.
In the first semiconductor device of the invention, since Bi is included by an excessive amount corresponding to a composition ratio y (y greater than 0) of the general formula (1), even though Bi escapes from the crystal structure with the elapse of time, the excessive Bi enters a portion from which Bi has escaped, and therefore, the characteristic of the ferroelectric film is never degraded. Also, the Bi excessive content y satisfies a relationship of yxe2x89xa6(4xe2x88x92x)xc3x970.1, a leakage current flowing due to deposition of bismuth on the ferroelectric film can be prevented. Accordingly, the reliability of the semiconductor device can be improved.
Preferably, in the first semiconductor device of the invention, (4xe2x88x92x)xc3x970.02xe2x89xa6yxe2x89xa6(4xe2x88x92x)xc3x970.06 in the general formula (1).
Thus, the initial polarization value of the ferroelectric film can be 20 xcexcC/cm2 or more, and hence, the reliability of the ferroelectric film, namely, the reliability of the semiconductor device, can be improved.
Preferably, in the first semiconductor device of the invention, A is La and 0.65xe2x89xa6xxe2x89xa60.85 in the general formula (1).
Thus, the initial polarization value of the ferroelectric film can be 20 xcexcC/cm2 or more, and hence, the reliability of the ferroelectric film, namely, the reliability of the semiconductor device, can be improved.
The second semiconductor device of this invention includes a ferroelectric film; and an electrode provided in contact with at least one face of the ferroelectric film, and the ferroelectric film is made from a ferroelectric material represented by a general formula (2), (Bi4xe2x88x92x+yAxTi3O12)z+(DBi2E2O9)1xe2x88x92z, wherein A is an element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and V; D is an element selected from the group consisting of Sr, Ba, Ca, Bi, Cd, Pd and La; E is an element selected from the group consisting of Ti, Ta, Hf, W, Nb, Zr and Cr; and 0xe2x89xa6xxe2x89xa62, 0 less than yxe2x89xa6(4xe2x88x92x)xc3x970.1 and 0.5 less than z less than 1.
Since the second semiconductor device of the invention uses the second ferroelectric film of the invention, the reliability of the ferroelectric film, namely, the reliability of the semiconductor device, can be improved and a low-voltage operating characteristic can be realized.
The third semiconductor device of this invention includes a ferroelectric film; and an electrode provided in contact with at least one face of the ferroelectric film, and the ferroelectric film is made from a ferroelectric material represented by a general formula (3), Bi4xe2x88x92x+yxe2x88x92rAxDrTi3xe2x88x92sEsO12, wherein A is an element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and V; D is an element selected from the group consisting of Sr, Ba, Ca, Cd, Pb and La; E is an element selected from the group consisting of Ta, Hf, W, Nb, Zr and Cr; and 0xe2x89xa6xxe2x89xa62, 0 less than yxe2x89xa6(4xe2x88x92x)xc3x970.1, 0 less than x+rxe2x89xa62, 0xe2x89xa6rxe2x89xa60.5 and 0xe2x89xa6sxe2x89xa60.5.
Since the third semiconductor device of the invention uses the third ferroelectric film of the invention, the reliability of the ferroelectric film, namely, the reliability of the semiconductor device, can be improved and the low-voltage operating characteristic can be realized.
In any of the first through third semiconductor devices of the invention, the electrode is preferably made from at least one of iridium and iridium oxide.
Thus, there is no need to provide a complicated oxygen barrier layer, and therefore, a ferroelectric capacitor can be downsized and capacitor fabrication process can be simplified because the electrode structure can be simplified